1. Field of the Invention
The present invention relates to an optical pickup device used for recording an information signal on an optical disk and reproducing the information signal recorded on the optical disk and an optical disk apparatus using the optical pickup device.
2. Description of the Related Art
There has been used an optical pickup device that records an information signal on and/or reproduces the information signal from optical disks such as a CD (Compact Disc) and a DVD (Digital Versatile Disc) and optical disks that make it possible to perform high-density recording.
Such an optical pickup device is desired to be reduced in size and simplified to be applicable to various uses. The reduction in size and the simplification of the optical pickup device can be realized by setting a total track length of an optical system short, i.e., setting a total length of the optical system short.
The total track length of the optical pickup device mainly depends on a media standard of an optical disk, on which an information signal is recorded and from which the information signal is reproduced, and a characteristic of an angle of divergence of a semiconductor laser used by the optical pickup device. The optical pickup device of the past converts a light beam emitted at a predetermined angle of divergence from a light source such as a semiconductor laser into parallel light using a collimator lens. The optical pickup device condenses the light beam, which is converted into the parallel light, on the optical disk using an object lens having a predetermined numerical aperture to form a spot.
In the optical pickup device of such a structure, when a media standard is determined, a numerical aperture of the object lens dependent on a spot size and RIM intensity of a light beam made incident on the object lens are determined. The RIM intensity is a ratio of intensity in the outer periphery to intensity in the center of the object lens.
On the other hand, in order to use an area of an appropriate intensity distribution with respect to the angle of divergence of the semiconductor laser, a focal length of the collimator lens is determined. Consequently, a magnification of the optical system of the optical pickup device is determined and, at the same time, the total track length is restricted by focal lengths of the object lens and the collimator lens.
It is possible to secure predetermined RIM intensity of the light beam made incident to the object lens and set the total track length, which is determined by the restriction, short by using an divergence-angle converting leans that converts an angle of divergence of a light beam emitted from the light source rather than using the collimator lens.
Specifically, the divergence-angle converting lens has an optical arrangement of a retro-focus type that can diverge the light beam emitted from the light source at a larger angle of divergence when the light beam is made incident on a first surface on an incidence side of the lens and emit the light beam at a predetermined angle of divergence when the light beam is emitted from a second surface on an emission side of the lens. Moreover, the divergence-angle converting lens is formed by combining object lenses having finite magnifications corresponding to the lens. Therefore, it is possible to set the total track length short while securing the predetermined RIM intensity and the predetermined numerical aperture.
However, usually, when the object lens and the divergence-angle converting lens are designed under an aplanat condition without a spherical aberration and a comatic aberration to form an optical pickup device, astigmatism occurs when the object lens shakes a visual field thereof. When the total track length is set short as described above, an amount of astigmatism amount caused when the visual field is shaken increases and significantly affects signal detection.
As a related art, there is JP-A-2004-2478034.